1. Field of the Invention
The present invention generally relates to a rubber composition, and a rubber article, such as a rubber vibration isolator and a rubber tire, formed from the rubber composition. In particular, the present invention is concerned with a rubber composition suitable for producing rubber articles which possess not only high heat-resistance, high durability, but also excellent dynamic-spring characteristic (i.e., sufficiently low dynamic spring constant).
2. Discussion of the Prior Art
Rubber articles have been used as a vibration isolator which is interposed between two members in a vibration transmission system so as to connect the two members in a vibration-isolating manner. In an automotive vehicle, for example, a large number of rubber vibration isolators are employed for improving ride comfort characteristic, driving stability, etc.
These rubber vibration isolators are required to possess specific physical and chemical properties according to locations at which the isolators are disposed. In particular, automotive vibration isolators such as an engine mount are required to exhibit a sufficiently low dynamic spring constant, upon application thereto of high-frequency vibrations, so as to achieve their specific purpose of preventing generation of booming noises in the passenger's room. In addition, the automotive vibration isolators must have a satisfactory durability so as to be able to serve for a long period. Furthermore, in recent years, the automotive vibration isolators are required to possess a high heat-resistance because there is the tendency that both the engine room and the exhaust emission system are subject to raised temperatures due to the use of a high-performance and high-power engine and the regulations on exhaust gas, respectively.
Also, rubber tires such as an automotive wheel tire are required to possess the same characteristics as indicated above concerning the rubber vibration isolators.
For producing rubber formed articles, such as a vibration isolator and a wheel tire, which have a sufficiently low dynamic spring constant, it is usually carried out to use sulfur serving as a vulcanizing agent, in an increased amount, or employ butadiene rubber (BR). Alternatively, it has conventionally been carried out to use carbon black serving as a reinforcing agent, in a reduced amount. However, the use of increased amount of sulfur tends to lower heat-resistance of the rubber products, and the use of butadiene rubber or use of reduced amount of carbon black tends to lower durability of the rubber products.
Meanwhile, for improving heat-resistance of rubber articles, it is usually performed to reduce the amount of sulfur used, to the lowest level that permits cross-linking of the rubber material (this method is called the "EV (effective vulcanization)" method), or reduce the sulfur amount to a level between the levels employed in the common, sulfur vulcanization and the EV method (this method is called the "semi-EV (semi-effective vulcanization)" method). Alternatively, a heat-resistant polymer is employed. However, in any case, the produced rubber articles exhibits an excessively high dynamic spring constant, and the use of heat-resistant polymer, in particular, tends to deteriorate durability of the rubber products.
Furthermore, for improving durability of rubber articles, it is generally carried out to use an increased amount of carbon black, or select a specific carbon black which has a higher reinforcing property. In either case, however, the produced rubber articles have an excessively high dynamic spring constant. In particular, in the case of a carbon black, such as HAF (high abrasion furnace black), whose particle size (diameter) is relatively small, the dynamic spring constant of the end products varies depending upon frequencies of the vibrations applied thereto. More specifically described, the dynamic spring constant of the rubber articles produced using such a carbon black, becomes extremely high upon application thereto of high-frequency vibrations (this effect is called the "Payne" effect).
As is apparent from the foregoing, the above-indicated conventional techniques have not been satisfactory to produce rubber articles with not only high heat-resistance, high durability, but also excellent dynamic-spring characteristic or a sufficiently low spring constant. Consequently, desired rubber vibration isolators or desired rubber wheel tires have not been provided yet.